My guess is that background processes are holding it up or your WiFi connection need some adjusting. You may want to check the link in my sig for WiFi tuneup suggestions.
I followed Leeshor's Wifi suggestions and I have a very good connection with my cable router and I still have a 2 or 3 second run and then YouTube stops and I hit play again and sometimes it plays all the way through and sometimes it stops and I have to start it several times to get to play all of the video.